By Carol Fey

A boiler water temperature controller is just an on and off switch that controls water temperature. How simple can you get? It’s like a thermostat, but for water instead of air. Yet the terminology associated with the controllers — differential, switching action, immersion, capillary — creates the impression that they’re complex.

The sense of difficulty is heightened when you try to use the part number on a controller to find an exact replacement. It just about can’t be done, because there are hundreds of different part numbers for very similar controls. Even exemplary supply houses often don’t have access to a complete part number cross-reference. However, if you’re willing to identify a few specifications, you can use a generic replacement with confidence.

Here’s seven things you need to know to find the perfect replacement:

  1. Bulb Type
    The temperature sensor is called a bulb. The three types to choose from have to do with how the sensor is mounted. The most accurate is an immersion bulb. It’s mounted either directly into the boiler water, or into a well that allows the bulb to be replaced without draining the boiler. A strap-on bulb is more convenient to install. It’s mounted outside the boiler, often on piping. Because it’s not in direct contact with the water, temperature sensing may be less precise. When the bulb and the controller need to be a distance apart from each other, a remote bulb can be used. It has a capillary tube that is several feet long.
  2. Capillary Length
    The capillary is the tube that connects the sensing bulb to the switch in the controller. It’s filled with a substance that expands as it warms and exerts pressure upon the switch. This is how a change in water temperature results in switching action. Your only concern is to be sure the capillary tube is long enough.
  3. Operating Range
    The controller operating range needs to include the temperatures at which you want to maintain the boiler water. An example is a range of 100F to 240F.
  4. Differential
    If you tried to maintain one exact temperature, the burner would turn on and off constantly. Instead, there’s a range of acceptable temperatures called differential. This is why the boiler water isn’t always the same temperature. The bigger the differential, the less often the boiler will fire — and it will run longer when it does. The controller that originally comes with a boiler often will have a “fixed” differential, for example 10 degrees. For versatility, replacement controllers often have an adjustable differential, such as 10 to 25 degrees. You select from within that range when you install it.
  5. Application
    There can be several different purposes for a water temperature controller. Used as a high limit, the controller turns off the boiler burner when the water temperature reaches its highest permissible temperature. Functioning as a circulator control, the controller turns on the circulator when boiler water temperature reaches a particular temperature. As a low limit, the controller brings on the burner to heat the water.
  6. Reset
    The term reset refers to what the control does after it reaches the high limit. Automatic reset means that the burner can come on again as soon as the water temperature drops. Manual reset is for situations with safety concerns. It requires that a person push the controller-reset button before the burner can function again.
  7. Switching Action
    You may see the controller description, “contacts make on temperature rise,” or “contacts break on temperature rise.” The term “contacts” means switch. When a switch “makes,” it closes or turns on. When the water temperature rises to set point, the switch turns something on. This could be a circulator. When the water is warm enough, the circulator comes on to move the water to deliver heat. When a switch “breaks,” it opens or turns off.

    “Break on temperature rise” would be useful for a high limit. When the water temperature rises to set point, the switch turns off the burner.

You can choose a controller that has just one of these functions or more. Single function is the same as an ordinary household light switch. You can turn one item either on or off. This is called SPST (single pole, single throw).

Multiple function controllers have a combination of functions such as high limit and low limit, high limit and circulator control or high limit, low limit and circulator control. Then the switching is called SPDT (single pole, double throw). Double throw allows one thing to be turned off at the same time that another is turned on and vice versa.

Electronic temperature controllers do the same thing as the old fashioned type. An electronic thermistor is inside the bulb rather than heatsensitive fill material. Instead of a capillary tube, electrical wire sends a resistance signal from the thermistor to the controller switch.

In the hydronics industry we know boiler water temperature controllers as Aquastats. It’s a little known fact that an Aquastat is a trademark. But whether you call it an Aquastat or a water temperature controller, it’s still just a simple switch.

Carol Fey is a technical trainer who has spent more than 25 years in the heating and controls industry. She has published four books and a DVD in the “Quick & Basic” series, including Quick & Basic Hydronic Controls. She can be reached at carol@carolfey.com or on her website, www.carolfey.com.